WICKET-4268 Quality of ThumbnailImageResource

Make #getScaledImageInstance() non-final so client applications can provide better algorithm for scaling

(cherry picked from commit d1e5a7494277cb1eb91c4615e3a470bf54b91fda)
Fix a minor typo in the comments

(cherry picked from commit e624c74d74fd273c4080c23eedd74f4e02998745)